In steam generating systems, a condenser is used downstream of a steam turbine to convert steam, after it has passed through the steam turbine, from its gaseous state to its liquid state. The condenser may be air-cooled and comprises a steam inlet duct, a plurality of condenser tubes, and a condensate outlet duct. Steam passes into the condenser through the steam inlet duct and flows through the condenser tubes. Air is forced over outer surfaces of the tubes so as to cool the tubes and, hence, the steam flowing through the tubes, thus causing the steam to be converted into a liquid condensate. The condensate is reused in generating steam for the steam turbine such that at least a portion of it later returns to the condenser where it is once again converted to its liquid state in the condenser.
It is desirable to prevent contaminants, such as oxygen and carbon dioxide, from entering the condenser. When the concentrations of oxygen and carbon dioxide are high enough, they become corrodents to iron used in the condenser and other components of the steam generating system, including piping and a steam generator. The corrosion product is iron oxide which tends to deposit on the steam generator surfaces and reduce heat transfer. Corrosion also causes wall thinning of the condenser tubes and other steel structures, and can result in leaks and failures. In addition to being a corrodent, carbon dioxide interferes with monitoring of the steam generating system for more corrosive species, such as chloride. Hence, carbon dioxide is a nuisance that may require the steam generating system to use more sophisticated monitoring equipment at significantly greater expense.
The normal operating pressure in the condenser may be a few inches of mercury (absolute pressure) and, hence, the normal operating pressure is at a vacuum, i.e., less than 1 atmosphere absolute pressure. To prevent contaminants from entering the condenser after steam turbine shut-down, it is known to “break vacuum” with nitrogen instead of air. To “break vacuum” is to admit gas into the condenser in order to raise the pressure within the condenser from vacuum to a pressure substantially equal to 1 atmosphere. When nitrogen is used, the pressure is slightly above 1 atmosphere to be sure that leaks flow from the condenser to the outside. While this process reduces air ingress into the condenser after steam turbine shut-down, it is not completely effective because the condenser usually starts and operates below 1 atmosphere pressure and air leaks in while the condenser is below 1 atmosphere.